One of the things I like in this game is how you can lose slowly (and yes, the thing that pushes my buttons most is losing).
I love not quite inescapable yet very stacked situations. Multiple turns where I don't have green to escape a draining kiss, for example. What I'm not such a fan of is Slimantha bitchslapping me to death in 3 turns flat when we're at neutral size.
So short version is I'd like more of the former, less of the latter, YMMV ofc.
As for how to achieve that (maybe on a future enemy as I think your enemy design is already mostly set for the current two) - I'd suggest adding mechanics that can be foreseen a little in advance.
For example, say a warning card gets played 3 turns (or next deck cycle) before and the player has to meet some requirement before the end of that turn. Then failing to prevent it puts a long term (5-15 turns) debuff on.
The debuff should be something disruptive on it's own that can become overwhelming if stacked (like the slow down debuff level of power, or shop price increases or discard every x cards played) (but you'd want various different effects with different triggers, making it tricky to keep track of).
This can hopefully lead the player to a downwards spiral of debuffs making it harder to meet the requirements for future debuffs and eventually the tiny player gets permanent debuffs applied and is slowly brought to defeat.